In lupus nephritis (LuN) renal biopsies are used to assess the extent of renal involvement, assign prognosis and to aid in making therapeutic decisions. We and other groups have demonstrated that the degree of tubulointerstitial inflammation (TH) and scarring, independent of co-variance with glomerular pathological features, predict progression to renal failure. As demonstrated in Preliminary Results, within the inflamed tubulointerstitium there was selection for repertoires of antibodies that were specific for molecules associated with inflammation including vimentin which coated the surfaces of infiltrating T cells and macrophages. These results suggest a model in which in situ adaptive immunity amplifies inflammation. While these findings identified the antigens driving in situ selection, it was not clear what co-stimulatory signals were also contributing to in situ B cell activation. With the development of novel quantitative imaging methods (Cell Distance Mapping and Analysis, CDMA, Preliminary Results), we were able to demonstrate that T follicular helper cells (TFH) cells were in tight cognate pairs with B cells. TFH-like cells and B cells were observed in the Til associated with both renal allograft T cell mediated rejection (TCMR) and mixed rejection (MR). However, in MR there were TFH-dependent and independent B cell populations while in TCMR the TFH cells appeared incompetent to form conjugates with B cells. These and other data indicate that we can identify functionally different populations of both TFH and B cells in situ. We hypothesize those T cells able to form cognate pairs with B cells in situ will have the molecular features of mature TFH cells. Furthermore, we hypothesize that the molecular signatures of competent TFH cells will be characteristic of each disease. Finally, we hypothesize that for each disease, and for each TFH cell population, different B cell functional repertoires will be selected. These hypotheses will be tested in the following Specific Aims: Aim 1. Determine the B cell repertoire selected in situ by TFH cells in lupus nephritis. Aim 2. To characterize the repertoire of B cells that are, and are not, selected by TFH in situ. Aim 3. Understanding the role of T cell help in in situ autoimmunity.